Optical disk and method of manufacturing the same

ABSTRACT

An optical disk having a recording layer on a substrate, a bonding layer which is made of an ultraviolet cured resin bonding the recording layer, a transparent layer which is bonded to the bonding layer, and an overcoating layer which is formed on the transparent sheet and is made of the ultraviolet cured resin further has a mixed layer which is formed by mixing the ultraviolet cured resins of the bonding layer and the overcoating layer. Consequently, the bonding layer, the overcoating layer, and the mixed layer are integrally cured, so that the substrate is entirely covered with the ultraviolet cured resin having transparency, which allows a light transmitting layer A to be protected with sufficient strength.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending U.S. application Ser. No.11/889,652, filed Aug. 15, 2007, and for which priority is claimed under35 U.S.C. §121. U.S. application Ser. No. 11/889,652, is a Divisional ofU.S. application Ser. No. 11/430,930, filed May 10, 2006, now U.S. Pat.No. 7,318,233, and for which priority is claimed under 35 U.S.C. §121.U.S. application Ser. No. 11/430,930 is a divisional of U.S. applicationSer. No. 10/448,213, filed May 30, 2003, now U.S. Pat. No. 7,062,776,and for which priority is claimed under 35 U.S.C. §121. This applicationis based upon and claims the benefit of priority under 35 U.S.C. § 119from the prior Japanese Patent Application No. 2002-158897, filed May31, 2002. The entire contents of each of the above-identifiedapplications for which priority is claimed is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disk in which a laser beamis irradiated from a light transmitting layer side provided on asubstrate to record and reproduce information, particularly relates tothe optical disk in which a protective layer for the light transmittinglayer is provided and a method of manufacturing the same.

2. Description of the Related Art

Recently, an optical recording medium such as an optical disk isbecoming commonplace, for example, in DVD (Digital Versatile Disk) orDVD-R for a DVD apparatus, a laser beam is irradiated from a backside ofa substrate to record and reproduce information. However, in thenext-generation DVD apparatus or the like, recording and the reproducingare performed in such a manner that a recording layer is formed on theface of the substrate, a light transmitting layer is provided on therecording layer, and the laser beam is irradiated from a frontside ofthe disk.

That is, in the optical disk, the recording layer (a deposited layerwhich directly involves the recording, erasing, and reproducingcharacteristics of a phase change type of optical disk such as areflecting film, a dielectric material film, and a phase change film) isformed on a transparent plastic substrate in which a guide groove isformed by an injection molding method or the substrate made by a so-call2P method, and the light transmitting layer (a bonding film made ofultraviolet cured resin and a transparent sheet) is formed on therecording layer. Usually, the recording layer is formed by a sputteringmethod, and the light transmitting layer is formed in such a manner thatresin liquid is applied on the substrate and the recording layer with aspin coat method and is hardened, and the transparent sheet is fixed.

However, when the light transmitting layer is formed up to an outermostperipheral end of the substrate, external force of handling causes aproblem such as easy separation. The reason is as follows. That is, itis necessary that the light transmitting layer is larger than therecording layer in order to protect the recording layer. The lighttransmitting layer has a thickness of about 100 μm and requires to becovered with the transparent sheet having excellent evenness, because itis necessary that birefringence is decreased and the thickness isuniformed on the entire periphery surface of the substrate. However, inthe case of adopting the above-described structure, since the outermostperipheral end portion of the transparent sheet constituting the lighttransmitting layer is as thick as 100 μm, it is easy to generate a stepat the outermost peripheral end portion of the substrate. In some cases,the transparent sheet is separated by getting hitched during thehandling, so that the recording layer has a possibility of being broken.

With reference to the method of manufacturing an optical disk, inJapanese Patent. Application KOKAI Publication No. 5-81701 which is areference disclosing the technique forming the protective layer forprotecting a given layer, there is disclosed a technique in which theprotective layer and the resin layer are formed on a metal reflectinglayer in order to prevent the metal reflecting layer from beingseparated from the substrate. In the optical disk, the laser beam isirradiated from the backside of the substrate to the recording layer andto record and reproduce the information.

However, since the optical disk shown by the reference is not thenext-generation optical disk, the laser beam is irradiated from thebackside of the substrate to the recording layer. Accordingly, unlikethe optical disk according to one embodiment of the invention, thetechnique in which the light transmitting layer including thetransparent sheet and a bonding layer is bonded to the substrate and isprotected with sufficient strength, while transparency is retained, isnot disclosed. Therefore, there is a problem that the light transmittinglayer of the optical disk, in which the recording and the reproducingare performed by irradiating the laser beam from the frontside of theoptical disk, can not be bonded and protected with the sufficientstrength.

That is, according to the conventional manufacturing method, in theoptical disk in which the recording and the reproducing are performed byirradiating the laser beam from the frontside of the optical disk, thereis the problem that, while transparency is retained, the lighttransmitting layer including the transparent sheet and the bonding layercan not be bonded onto the substrate and protected with the sufficientstrength.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the present invention refers to an optical disk, whichcomprises a recording layer in which information provided on a substrateis recorded, a bonding layer which is made of an ultraviolet cured resinbonding at least the recording layer onto the substrate, a transparentsheet layer which is provided to be bonded on the bonding layer, anovercoating layer which is made of the ultraviolet cured resin and isformed on the transparent sheet layer, and a mixed layer which is formedcontinuously to the overcoating layer by mixture of the ultravioletcured resins of the bonding layer and the overcoating layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a longitudinal sectional view showing a first embodiment of anoptical disk according to one embodiment of the invention;

FIG. 2 is a longitudinal sectional view showing a second embodiment ofthe optical disk according to one embodiment of the invention;

FIG. 3 is a longitudinal sectional view showing a third embodiment ofthe optical disk according to one embodiment of the invention;

FIG. 4 is a longitudinal sectional view showing a fourth embodiment ofthe optical disk according to one embodiment of the invention;

FIG. 5 is a top view showing a fifth embodiment of the optical diskaccording to one embodiment of the invention;

FIG. 6 is a top view showing a sixth embodiment of the optical diskaccording to one embodiment of the invention;

FIG. 7 is a sectional view showing a fifth embodiment of the opticaldisk according to one embodiment of the invention;

FIG. 8 is a sectional view showing a sixth embodiment of the opticaldisk according to one embodiment of the invention;

FIG. 9 is a longitudinal sectional view of a first optical disk which isa comparative example of the optical disk according to one embodiment ofthe invention;

FIG. 10 is a longitudinal sectional view of a second optical disk whichis a comparative example of the optical disk according to one embodimentof the invention;

FIG. 11 is a longitudinal sectional view of a third optical disk whichis a comparative example of the optical disk according to one embodimentof the invention; and

FIG. 12 is a flow chart explaining a process for manufacturing theoptical disk according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, an optical disk and a method of manufacturingan optical disk will be described in detail below.

First Embodiment

A first embodiment provides the optical disk in which the lighttransmitting layer, which includes a bonding layer and a transparentsheet smaller than the substrate provided on the optical disk, is bondedand protected by the bonding layer, the overcoating layer, and the mixedlayer of these materials, which are made of the ultraviolet cured resin.FIG. 1 is the longitudinal sectional view showing the first embodimentof the optical disk according to one embodiment of the invention, andthe FIG. 9 is the longitudinal sectional view of a first optical diskwhich is a comparative example of the optical disk according to oneembodiment of the invention.

FIG. 9 shows the optical disk in which the overcoating layer and themixed layer according to one embodiment of the invention are notprovided. In the optical disk shown in FIG. 9, an outer peripheralbonding face 7 of a substrate 1 and a transparent sheet 4 is liable tothe separation from deformation of the substrate 1 and the transparentsheet 4, linear expansion, or plane shrinkage. Particularly thisphenomenon tends to occur in high temperature and high humidity. This isbecause the materials of the substrate 1 and the protective layer differin heat distortion properties, linear expansion coefficient,heat-shrinkable properties, and hygroscopic properties (water absorptioncoefficient). Further, since a distance from the outermost peripheraledge of the substrate 1 to a recording region is as very short as 1 mm,when once there is a portion where adhesion between the substrate 1 anda recording layer 2 is slightly bad, the separation propagates from theportion to the recording region to destroy a recording layer 2.

When the transparent sheet 4 having an about 100 μm thickness is bondedonly by the ultraviolet cured resin, a step is easily generated in theoutermost peripheral edge portion of the transparent sheet 4. On theother hand, when a crease of the sheet and the like in die-cutting aregenerated at an outer diameter edge face of the transparent sheet 4,adhesion is easily incomplete. This tends to result in intrusion of airand imperfect adhesion of the transparent sheet 4. In such case, sincethe protection of the recording layer 2 is not sufficiently performeddue to the short distance to the recording region, the recording layer 2is more liable to the separation. When regression caused by theseparation is remarkable, the recording layer 2 is not completelycovered by the transparent sheet 4 of the light transmitting layer, andsometimes part of the recording layer is exposed in air, so thatdurability of the recording layer 2 becomes insufficient.

In the optical disk according to the first embodiment of the invention,as shown in FIG. 1, an overcoating layer 5 made of the ultraviolet curedresin is formed on the transparent sheet 4, and a mixed layer 6including the bonding layer 3 and the overcoating layer 5 is provided atleast in the sectional face of the bonding layer 3 and a side face ofthe substrate. Herein, the transparent sheet 4 having the smallerdiameter than that of the substrate 1 is used.

As described above, since both the bonding layer 3 and the overcoatinglayer 5 are the ultraviolet cured resin, the mixed layer 6 is easilyformed in a mixed portion of the bonding layer 3 and the overcoatinglayer 5. The bonding layer 3, the overcoating layer 5, and the mixedlayer 6 are integrally formed by the continuous ultraviolet cured resin,so that a bonded face “G” of the substrate 1 and the recording layer 2is covered with mixed layer 6, and the recording layer 2 and the lighttransmitting layer “A” including the bonding layer 3 and the transparentsheet 4 are bonded with each other and are protected with extremely highstrength. This was sufficiently confirmed by a durability test describedlater.

(Method of Manufacturing Optical Disk According to One Embodiment of theInvention)

A method of manufacturing an optical disk according to one embodiment ofthe invention will be described in detail below by the use of a flowchart. FIG. 12 is the flow chart explaining a process for manufacturingthe optical disk according to one embodiment of the invention.

In the optical disk according to one embodiment of the invention, thetransparent substrate 1 (made of polycarbonate resin, the 120 mmdiameter and the 1.1 mm thickness), which had the 1-mm-width and0.5-mm-depth depression at the 22 mm diameter and the guide groove, wasprepared in the flow chart shown in FIG. 12 (S1). Then, the recordinglayer 2 was formed on the substrate 1 with a magnetron sputteringapparatus. A metal reflecting film made of an AlMo alloy, a dielectricprotective film made of ZnS and SiO₂, a phase change material film madeof a GeSbTe alloy, and the above-described dielectric protective filmare formed sequentially as the recording layer 2. Composition of thephase change material film is formed such that the composition has thegood recording, erasing, and repeated recording/reproducingcharacteristics with the 405 nm wavelength and reflectivity differencebetween a crystal phase and an amorphous phase is enlarged. For themetal reflecting film, the composition is formed so as to have afunction of improving the reflectivity of the phase change optical disk.

At this point, a mask having the 36 mm diameter is used as an innerradius mask and the mask having the 119 mm diameter is used as an outerradius mask. This is because the recording region is provided up to theneighborhood of the inner and outer radiuses so as not to reducerecording capacity as much as possible.

Then, ultraviolet cured type of adhesives 11 was applied onto thesubstrate 1 on which the recording layer 2 had been formed with a spincoating method (S2). At this point, viscosity of the adhesives 11 wasset to N_(a) by way of example. The previously prepared, doughnut-shapedtransparent sheet 4 for light transmission having the 119 mm outerdiameter, the 16 mm inner diameter, and the 0.1 mm thickness, which wasslightly smaller than the substrate 1, was placed on the appliedultraviolet-cured type of adhesives 11 (S3). Immediately, the bondinglayer 3 was formed between the recording layer 2 and the transparentsheet 4 by rotating a spindle of a spin coater to drain off the excessadhesives (S4). At this point, the adhesives 11 of the bonding layer 3had an overflow portion 12.

Successively, an ultraviolet cured type of overcoating agent 13 wasapplied on the transparent sheet 4, which was fixed by the bonding layer3, with the spin coating method (S5). At this point, by way of example,the viscosity of the overcoating agent 13 was set to the viscosity N_(b)which was lower than that of the previous adhesives 11, however, theequal viscosity N_(a) was also applicable to the overcoating agent 13.The overcoating layer 5 was formed on the transparent sheet 4, which wasfixed by the bonding layer 3, by rotating a spindle of a spin coater todrain off the excess overcoating agent. Because the adhesives of thetransparent sheet 4 for light transmission were also not cured, theadhesives were mixed with the overcoating agent in the same non-curedstate at the outer peripheral portion, which resulted in the mixed layer6 (S6). Then, by irradiating an ultraviolet ray from the side of theovercoating layer 5, the bonding layer and overcoating layer side 5 werecured to prepare the phase change optical disk (S7). The thicknesses ofthe bonding layer and the overcoating layer 5 were formed after curingin the range from 2 to 5 μm by controlling the number of revolutions ofa turntable.

As a result, in the outer peripheral portion of the disk, the layer inwhich the adhesives and the overcoating agent are mixed is formed so asto go round to the side face of the substrate 1. Accordingly, thebonding layer 3, the overcoating layer 5, and the mixed layer 6 areintegrated and cured with one ultraviolet irradiation, so that thesubstrate 1 is firmly protected by the ultraviolet cured resin which hasbeen cured. As a result, the light transmitting layer “A” including thetransparent layer 4 and the bonding layer 3 is protected with extremelyhigh strength, it is sufficiently confirmed by the durability testdescribed later.

It is preferable by way of example that the thickness of the opticaldisk is in the range from 1.0 mm to 1.2 mm and the thickness of thetransparent sheet is in the range from 50 μm to 110 μm.

Second Embodiment

A second embodiment provides the optical disk in which the lighttransmitting layer, which includes the bonding layer and the transparentsheet having the same diameter as that of the substrate provided on theoptical disk, is bonded and protected by the bonding layer, theovercoating layer, and the mixed layer of these materials, which aremade of the ultraviolet cured resin. FIG. 2 is the longitudinal viewshowing the second embodiment of the optical disk according to oneembodiment of the invention.

In FIG. 2, the phase change optical disk was prepared in the same way asthe first embodiment except that the transparent sheet 4 for lighttransmission having the same outer diameter of 120 mm as the substrate 1was used. As a result, in most disks, the mixed layer in which theadhesives and the overcoating agent were mixed in the outer peripheralportion was formed so as to go round to the side face of the substrate1.

Consequently, similarly to the first embodiment, the bonding layer 3,the overcoating layer 5, and the mixed layer 6 are integrally formed, sothat the light transmitting layer “A” including the transparent layer 4and the bonding layer 3 and the recording layer 2 can be bonded andprotected with extremely high strength.

Third Embodiment

A third embodiment provides the optical disk in which the lighttransmitting layer, which includes the bonding layer and the transparentsheet having the larger diameter than that of the substrate provided onthe optical disk, is bonded and protected by the bonding layer, theovercoating layer, and the mixed layer of these materials, which aremade of the ultraviolet cured resin. FIG. 3 is the longitudinal viewshowing the third embodiment of the optical disk according to oneembodiment of the invention.

In FIG. 3, the phase change optical disk was prepared in the same way asthe first embodiment except that the transparent sheet 4 for lighttransmission having the larger outer diameter of 121 mm than that of thesubstrate 1 was used. As a result, in most disks, the mixed layer inwhich the adhesives and the overcoating agent were mixed in the outerperipheral portion was formed so as to go round to the side face of thesubstrate 1. In the side face portion of the substrate 1, the projectedportion of the transparent sheet 4 for light transmission is buried withthe layer in which the adhesives and the overcoating agent are mixed.

Consequently, similarly to the first embodiment, the bonding layer 3,the overcoating layer 5, and the mixed layer 6 are integrally formed, sothat the light transmitting layer “A” including the transparent sheet 4and the bonding layer 3 and the recording layer 2 can be bonded andprotected with extremely high strength.

Fourth Embodiment

A fourth embodiment provides the optical disk in which the side face ofthe substrate in the optical disk of the first embodiment, in which thelight transmitting layer including the bonding layer and the transparentsheet smaller than the substrate provided on the optical disk is bondedand protected by the bonding layer, the overcoating layer, and the mixedlayer of these materials which are made of the ultraviolet cured resin,is wiped off with a dust-free wiper to remove the mixed layer. FIG. 4 isthe longitudinal view showing the fourth embodiment of the optical diskaccording to one embodiment of the invention.

In FIG. 4, the phase change optical disk was prepared in the same way asthe first embodiment except that the side face of the substrate 1 waswiped off with the dust-free wiper after the overcoating agent layer wasformed. In the side face of the outer peripheral portion of the disk,the adhesives and the overcoating agent were wiped off clean and therewas no change in the shape such as the step.

Consequently, similarly to the first embodiment, the bonding layer 3,the overcoating layer 5, and the mixed layer 6 are integrally formed, sothat the light transmitting layer “A” including the transparent layer 4and the bonding layer 3 and the recording layer 2 can be bonded andprotected with extremely high strength.

Fifth Embodiment

A fifth embodiment provides the optical disk, in which the separationcaused by a touch of a shaft of a disk driver is avoided in such amanner that a central portion of the transparent sheet 4 provided on theoptical disk is partially cut and, in the same way as the firstembodiment and the like except that point, the optical disk is bondedand protected by the bonding layer, the overcoating layer, and the mixedlayer of these materials which are made of the ultraviolet cured resin.FIG. 5 is the top view showing the fifth embodiment of the optical diskaccording to one embodiment of the invention, and FIG. 7 is a sectionalview showing the fifth embodiment of the optical disk according to oneembodiment of the invention.

In these figures, after the phase change optical disk was prepared inthe same way as the first embodiment, a portion “T” of the transparentsheet 4, which had the diameter ranging from 16 mm to 21, was cut off,ultraviolet cured type of protective agent was applied in an end face,and the protective agent was cured with the ultraviolet irradiation toform the protective layer 7. As a result, the inner peripheral portionof the transparent sheet 4 is placed at more outside than that of thesubstrate 1, so that there is no sense of discomfort even if the innerperipheral portion is touched with a finger. In this way, by providingthe protective layer 7 which covers the inner peripheral end portion ofthe transparent sheet 4, the hitch caused by the touch of the shaft ofthe disk driver and the like can be released in the inner peripheralportion and the separation of the transparent sheet 4, which is causedby the hitch, can be prevented.

Sixth Embodiment

A sixth embodiment provides the optical disk, in which adhesion betweenthe optical disks is prevented to suppress the separation in such amanner that a ring-shaped sheet is provided as a spacer on the opticaldisk and, in the same way as the first embodiment and the like exceptthat point, the optical disk is bonded and protected by the bondinglayer, the overcoating layer, and the mixed layer of these materialswhich are made of the ultraviolet cured resin. FIG. 6 is the top viewshowing the sixth embodiment of the optical disk according to oneembodiment of the invention, and FIG. 8 is a sectional view showing thesixth embodiment of the optical disk according to one embodiment of theinvention.

In these figures, after the phase change optical disk was prepared inthe same way as the first embodiment, a sheet layer 8 having the38-mm-outer-diameter, the 35-mm-inner-diameter, and the 0.1-mm-thicknesswas concentrically bonded on the overcoating layer 5 by the adhesives.This enables the adhesion of each substrate to be prevented, even ifeach substrate is stacked in a stacking jig.

In the sixth embodiment, the ring-shaped sheet 6 is concentricallybonded to the optical disk with the adhesives, so that the adhesionbetween the optical disks can be prevented and the separation of thelight transmitting layer A can be prevented in the handling.

(Durability Test)

In order to confirm advantage of the optical disk according to oneembodiment of the invention, the durability test has been performed for(1) drop test and (2) environmental test in the optical disks accordingto the first to fourth embodiments and the corresponding optical disksof a comparative example 1 to a comparative example 3 in which theovercoating layer are not provided, as shown in FIGS. 9 to 11. Detailsof the comparative examples 1 to 3 and each test result are shown asfollows.

COMPARATIVE EXAMPLE 1

FIG. 9 is the longitudinal sectional view of a first optical disk whichis a comparative example of the optical disk according to one embodimentof the invention. In the first optical disk, the light transmittinglayer “A” was formed in the same way as the first embodiment except thatthe ultraviolet cured type of overcoating agent was not applied. As aresult, in the light transmitting layer “A”, the step was generated inthe outermost peripheral end of the substrate 1. Therefore, it was founda phenomenon in which the light transmitting layer A was easily liableto the separation from the hitch at the step portion in the handling.

COMPARATIVE EXAMPLE 2

FIG. 10 is the longitudinal sectional view of a second optical diskwhich is a comparative example of the optical disk according to oneembodiment of the invention. The phase change optical disk was formed inthe same way as the second embodiment except that the ultraviolet curedtype of overcoating agent was not applied. As a result, the end face ofthe outermost periphery in the light transmitting layer “A” was flushwith the end face of the outermost periphery of the substrate 1.Therefore, it was confirmed the phenomenon in which the lighttransmitting layer “A” was easily susceptible to the separation from thehitch at the end face portion of the outermost periphery in thetransparent sheet 4 in the handling, because the end face portion of theoutermost periphery in the transparent sheet 4 constituting the lighttransmitting layer “A” was an acute angle. Further, as shown in thesecond embodiment, since the end face of the outermost periphery in thelight transmitting layer “A” was not protected by the overcoating layer,the end face portion of the outermost periphery having the acute anglein the transparent sheet 4 constituting the light transmitting layer Awas exposed, so that it can be referred that safety was low in thehandling.

COMPARATIVE EXAMPLE 3

FIG. 11 is the longitudinal sectional view of a third optical disk whichis a comparative example of the optical disk according to one embodimentof the invention. The phase change optical disk was formed in the sameway as the third embodiment except that the ultraviolet cured type ofovercoating agent was not applied. At this point, the projected portionof the transparent sheet 4 was generated in the side face portion of thesubstrate 1. The shape shown in FIG. 11 referred to a state in which theseparation of the transparent sheet 4 was easier than that of the shapeshown in FIG. 9 because the hitch occurred easily at the projectedportion of the transparent sheet 4.

(1) Drop Test

High-impact resistance of the disk was examined in such a manner thatthe optical disks of the first to fourth embodiments and the opticaldisks of the first to third comparative examples were dropped from 1meter above the ground to a concrete floor so that the end face of theouter periphery was vertically hit.

The separation of the light transmitting layer “A” was not found in anyoptical disk among the optical disks of the first to fourth embodiments.On the other hand, in the optical disk of the comparative example 1, theseparation of the light transmitting layer A was found in the outerperipheral portion of the substrate 1, particularly in the optical disksof the second and third comparative examples, it was confirmed that thelight transmitting layer “A” became separated up to the recording regionof the recording layer 2 and the recording layer 2 was broken.

As described above, in the optical disks of the first to fourthembodiments according to one embodiment of the invention, it was able tobe confirmed from the experiment that the high-impact resistance wasgreatly improved.

(2) Environmental Test

The change in the shape of the optical disk was examined in such amanner that the environmental test at 80° C. and 90% RH for about 1000hours was conducted for the optical disks of the first to fourthembodiments and the optical disks of the first to third comparativeexamples.

The change in the shape was not found by a visible check in the opticaldisks of the first to fourth embodiments. On the other hand, in theoptical disk of the comparative example 1, the separation of the lighttransmitting layer “A” was found in the outer peripheral portion of thesubstrate 1, particularly in the optical disks of the second and thirdcomparative examples, it was confirmed that the separation of the lighttransmitting layer “A” proceeded up to the recording region of therecording layer 2, and corrosion of the recording layer 2 was confirmed.

As described above, in the optical disks of the first to fourthembodiments according to one embodiment of the invention, it was able tobe confirmed that corrosion resistance was greatly improved, comparedwith the optical disk of the first to third comparative examples.

Though those skilled in the art can realize the invention from variousembodiments described above, those skilled in the art can easilyconceive further various modifications of the embodiments and apply theinvention to various embodiments without inventive ability. Accordingly,the invention covers a wide range which is not contradictory to thedisclosed principle and novel characteristics, and is not limited to theabove-described embodiments.

For example, though the optical disk for the recording/reproducing wasdescribed in the above-described embodiments, the invention can also beapplied to the compact disk-read only memory. In this case, therecording layer 2 is formed by a simplex of the light reflecting filmmade of Al, an AlMo alloy, or the like.

As described above in detail, the invention provides the optical disk,in which the mixed layer in which the ultraviolet cured resins of thebonding layer of the transparent sheet and the overcoating layer aremixed, is formed, so that the bonding layer, the overcoating layer, andthe mixed layer are integrally cured. As a result, the substrate isentirely covered by the ultraviolet cured resin having the transparency,and the light transmitting layer including the transparent sheet and thebonding layer is protected with the sufficient strength.

The advantages of the high-impact resistance and the corrosionresistance of the invention were sufficiently confirmed by thecomparative experimental for (1) drop test and (2) environmental test.

1. An optical disc comprising: a substrate which has a center hole; arecording layer which is provided concentrically on the substrate, whosediameter is shorter than that of the substrate, and on which informationis recorded; a bonding layer which is provided on the recording layerand on the substrate so as to cover the recording layer, the bondinglayer being made of ultraviolet cured resin; a transparent sheet layerwhich is bonded onto the bonding layer and whose diameter is shorterthan that of the substrate; an overcoating layer which is formed on thetransparent sheet layer and which is made of ultraviolet cured resin;and a protective layer which is made of ultraviolet cured resin andwhich is formed to cover an end face of each of the recording layer, thebonding layer and the transparent sheet layer, which is closer to thecenter hole of the substrate.
 2. The optical disc according to claim 1,further comprising a materials-integration-layer which is formed on aside face of the substrate, the materials-integration-layer being formedintegrally with the side face of the substrate and extended portions ofthe bonding layer and the overcoating layer.
 3. The optical discaccording to claim 1, further comprising a ring-shaped layer whichprojects from the overcoating layer near the center hole of thesubstrate.
 4. An optical disc comprising: a substrate which has a centerhole; a light reflective layer which is provided concentrically on thesubstrate, whose diameter is shorter than that of the substrate, and onwhich information is recorded; a bonding layer which is provided on thelight reflective layer and on the substrate so as to cover the recordinglayer, the bonding layer being made of ultraviolet cured resin; atransparent sheet layer which is bonded onto the light reflective layerand whose diameter is shorter than that of the substrate; an overcoatinglayer which is formed on the transparent sheet layer and which is madeof ultraviolet cured resin; and a protective layer which is made ofultraviolet cured resin and which is formed to cover an end face of eachof the recording layer, the bonding layer and the transparent sheetlayer, which is closer to the center hole of the substrate.
 5. Theoptical disc according to claim 4, further comprising amaterials-integration-layer which is formed on a side face of thesubstrate, the materials-integration-layer being formed integrally withthe side face of the substrate and extended portions of the bondinglayer and the overcoating layer.
 6. The optical disc according to claim4, further comprising a ring-shaped layer which projects from theovercoating layer near the center hole of the substrate.